RAQQAH, Syria — As this remote corner of northeastern Syria fast slides out of government control, many Syrians are bracing for what they fear will be another war, between the relatively moderate fighters who first took up arms against the government and the Islamist extremists who emerged more recently with the muscle and firepower to drive the rebel advance.

The capture last month of the city of Raqqah, Syria’s first provincial capital to fall under opposition control, consolidated the gains of an assortment of mostly Islamist-inclined groups across three northeastern provinces. Forces loyal to President Bashar Assad cling to just a tiny number of scattered bases and could be ejected anytime.

Yet even as the regime continues to hold out, schisms are emerging among rebel groups over ideology, the shape of a future Syrian state and control of the significant resources concentrated in this long-neglected but crucial corner of the country.

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“Fighting is unavoidable,” said Abu Mansour, a commander with the rebel Free Syrian Army’s Farouq Brigades, whose men clashed last month with those of the extremist Jabhat al-Nusra movement in the border town of Tal Abiyad, one of several instances in which the tensions have erupted into violence. “If it doesn’t happen today, it will happen tomorrow.”

Jabhat al-Nusra, the group designated a terrorist organization by the United States because of its suspected ties to al-Qaida, is among several groups advancing in the region, but it is emerging as the most divisive and the strongest.

On Tuesday, the al-Qaida-affiliated Islamic State of Iraq announced that it had formally merged with Jabhat al-Nusra, with the two groups to be known jointly as the “Islamic State in Iraq and the Levant.”

There was no immediate confirmation of the alliance by Jabhat al-Nusra, which has often sought to portray itself as an indigenous Syrian organization and distance itself from the Iraqi organization, whose flag it shares. But the announcement underscored the potentially profound implications for Syria’s future of the fall of this northeastern region to the extremists.

The provinces of Raqqah, Deir al-Zour and Hasakah — collectively known by the ancient name of al-Jazeera, or the island, for their location between the Euphrates and Tigris rivers — are home to the bulk of Syria’s economic wealth, including all of its oil fields, as well as its gas reserves, and most of its agriculture, notably wheat and cotton.

The Jazeera region also reaches into the western Iraqi provinces of Nineveh and Anbar, where the Iraqi al-Qaida affiliate has its roots. Tribal and family ties span the border, and there are echoes of the complexity of the conflict that raged in Iraq in the past decade, when many Sunni tribesmen who initially joined the insurgency against U.S. troops switched sides and fought against al-Qaida.

Last week, a Saudi and two Tunisian fighters were killed when tribal leaders sought to prevent Jabhat al-Nusra fighters from entering the village of Misrib in Deir al-Zour. In Shahadi, an oil town in Hasakah province, Jabhat al-Nusra fighters opened fire on demonstrators protesting the group’s presence in the town on two occasions in the past month, a local activist said.

It is no accident, say more-moderate rebel leaders, that Jabhat al-Nusra has chosen to concentrate its efforts in this region. The group has seized control of nearly 90 percent of Syria’s oil wells, its granaries and its stores of cotton, and it has set about selling the stocks to raise money, according to Nawaf al-Bashir, a tribal leader. Bashir is a longtime regime opponent whose son was injured this month in a clash between the battalion he commands and Jabhat al-Nusra fighters in another Deir al-Zour village.

“They have the Syrian economy in their hands, and they are very strong. You can see their black flags everywhere you go,” he said in an interview in the Turkish border town of Sanliurfa.

Rumors swirl that tribes are hoping to form a “Sahwa,” or Awakening, movement similar to the one that the United States sponsored to quell al-Qaida in Iraq. Much of Hasakah province is inhabited by Kurds, many of whom aspire to independence and whose loyalties are divided, further complicating tensions in the region.

But most Syrians say they don’t want a fight, even as they acknowledge the growing divide. “Everyone knows what happened in Iraq, and we want to avoid that,” said Hamid al-Atullah, a spokesman for al-Jabhat al-Jazeera wa Furat, a coalition of rebel battalions formed partly to counter the influence of the radicals.

“The Syrian revolution started for democracy, and Jabhat al-Nusra are not fighting for democracy. But they are Syrians, and we don’t want any clash with them,” the spokesman added.

In the sleepy agricultural city of Raqqah, on the banks of the Euphrates, the reasons for Jabhat al-Nusra’s success are evident, as are the challenges the group confronts.

The homegrown Islamist group Ahrar al-Sham took the lead in the fighting here, with Jabhat al-Nusra units arriving only later. But they played a big part in securing the city’s infrastructure, businesses and shops against looting and in preventing the lawlessness that has stirred deep resentment toward the Free Syrian Army elsewhere.

Ahrar al-Sham’s local affiliate, the Liwa Umana al-Raqqah, runs a joint command center with Jabhat al-Nusra at the city’s sports stadium. The Liwa’s commander, Abu Tayf, a 45-year-old businessman from a rural suburb, describes himself as a moderate, says he wants Syria to become a democracy along the lines of Malaysia and Turkey, and urges the United States to implement a no-fly zone over Syria to halt the lethal daily airstrikes that remain the biggest source of fear for Raqqah’s residents.

But he also staunchly defends the role of Jabhat al-Nusra in the rebellion. If extremists are ascendant, “it is because America did nothing to help us,” he said. “If the revolution continues much longer and the world doesn’t help, I will join Jabhat al-Nusra.”

Former regime loyalists also appear to be embracing the extremists. As one of the last major cities to join the revolution and the first to expel the government, Raqqah was spared much of the fighting that has devastated other parts of the country. It also has a short history of opposition to the Assad regime. Residents who admit to supporting the government until recently appear seamlessly to have transferred allegiances to Jabhat al-Nusra, also known as al-Nusra Front.

“We were very afraid of Jabhat al-Nusra because the official media said they were terrorists, but it’s not true. They are good people,” said Miza Hussein, 37, a wealthy trader who has started contributing money to one of the group’s battalions. “I am too old to fight, but I want to be a part of them because I want equality and truth.”

“We just want to avoid conflict,” muttered one of his friends, among a group gathered in a shop selling wedding dresses on the main street of the city.

But Jabhat al-Nusra’s efforts to ban cigarettes as un-Islamic have not worked, nor have its efforts to compel all women to wear head scarves, even in a part of the country known for conservative traditions. “As you can see, we have freedom, and we are smoking,” said Wael Fouad, 27, who sells farming supplies nearby.

“No one is forcing anything on us,” said Hanin Mattar, 28, a teacher who walked past without a head scarf and said she had participated in the demonstrations against Assad. “And nor will they,” she added. “We fought for our freedom.”